3–10 keV Xe+ ion beam machining of ultra low thermal expansion glasses for EUVL projection optics: Evaluation of surface roughness

In order to obtain surface figure error of 0.15 nm rms and surface roughness (Rrms) of 0.12 nm rms for aspherical substrates in EUVL tools, ion beam figuring may be adopted to final surface figure error correction of aspherical substrates. During figure error correction, machined surface of the substrate becomes rougher than the pre-finished one. Therefore, we investigated the machined depth and ion energy dependences of Rrms (measured by an AFM) of substrates machined by 3–10 keV Xe+ ion beam, and compared them with the results obtained for Ar+ ion beam. Result shows that the RrmssRrmss of CLEARCERAM®-Z, Zerodur® and ULE® substrates machined to the depth of 50 nm by 3–10 keV Xe+ ion beam at the normal ion incidence angle become approximately 0.25, 0.28 and 0.15 nm rms, respectively. Those values are larger than the pre-finished substrates (0.07–0.09 nm rms), but smaller than that (0.60 nm rms for CLEARCERAM®-Z, 0.61 nm rms for Zerodur® and 0.18 nm rms for ULE®) of the substrates machined by Ar+ ion beam. Moreover, the RrmssRrmss merely increase with increasing ion energy. The RrmssRrmss of the ULE® substrate machined by 3–10 keV Xe+ ion beam rapidly increase with increasing machined depth, then saturate at machined depth of 10–50 nm. The saturated values of the RrmssRrmss are 0.12 and 0.15 nm rms for 3 and 10 keV Xe+ ion beam respectively. We suggest that the 3 keV Xe+ ion beam machining can be applicable for final shape correction of ULE® substrates for EUVL projection optics in association with considering further ultra smoothing process such as Si deposition or low energy ion beam smoothing.